This invention pertains to a load-clamping apparatus, and more particularly to such an apparatus wherein the loaded and unloaded condition of the clamping arms (respecting their contacting an external body) is automatically sensed, and used to determine the type of fluid connections which exist at any given moment between fluid motors that move the arms.
For a number of reasons, more and more lift trucks are being powered by electric batteries as distinguished from gasoline. Historically, hydraulically operated lift truck attachments have been designed to be compatible with the pressure and flow characteristics practically obtainable from hydraulic pumps driven by internal combustion engines. The trend toward increased use of electric lift trucks has created a need for more efficient hydraulic circuitry to operate such attachments, in order to take into account the substantially more time-consuming, and hence costly, process of recharging batteries in an electric lift truck, as distinguished from refueling of a gasoline powered truck. For example, if a gasoline-powered truck requires refueling during a typical 8-hour work shift, this is a matter which can be accomplished relatively quickly, and hence, relatively inexpensively. In other words, the downtime for such refueling is quite minimal. By contrast, however, recharging of batteries in an electric truck may take a considerable amount of time, and thus is to be avoided, if at all possible, during the period of a work shift. Accordingly, and in order to recognize the above-mentioned trend, as well as to recognize the general concern today for energy conservation, it is important to maximize the efficiency of hydraulic circuitry used in conjunction with lift-truck attachments, so as to make more conservative and efficient use of the power available in electric batteries.
An important object of the present invention, therefore, is to provide a unique hydraulic circuit usable in conjunction with a lift-truck attachment, such as a clamping mechanism, which circuit offers significantly greater efficiency than previously known circuits employed for the same general purpose.
More specifically, an object of the invention is to provide such a circuit wherein the loaded and unloaded conditions of clamping arms in a clamping mechanism (respecting their contacting an external body) is automatically sensed, and used to determine the nature of fluid connections which exist at any given moment between fluid motors that move the arms.
As will become apparent from the description below, when it is most appropriate that the arms move in what might be thought of as a high-speed, low-power mode of operation, connections are automatically produced between the motors and the main supply of hydraulic fluid which assure such action, with a minimal requirement for pumped pressure fluid to accomplish this. Similarly, when it is most appropriate that the arms move in what might be thought of as a low-speed, high-power mode of operation, different connections are produced between the motors which assure this action. Again, fluid flow to accomplish such action is held to a minimum.
The capability of the novel arrangement proposed herein to enable such different operating modes makes a maximum use of each unit amount of oil which must be pumped, and hence maximizes the efficiency of use of such oil. In addition, the proposed arrangement takes into account the fact that energy consumption from a battery is a function of the amount of amperage drawn from the battery, as well as the lengths of time that different amperages are drawn. Experience has shown that the proposed circuit when incorporated and used in conjunction with a conventional electrically powered lift truck, can enable normal, uninterrupted use of such a truck throughout the usual 8-hour work shift.
Disclosed herein are two related modifications of apparatus offering the features and advantages generally discussed above. These two modifications are disclosed in connection with two slightly different kinds of clamping needs often required for lift truck clamps. One of these modifications is referred to herein as a basic clamping apparatus which is usable simply for shifting a pair of opposed load-clamping arms toward and away from each other to clamp against and release a load. The other modification is referred to as a basic and side-shifting clamping apparatus, and is one which is usable not only for the same purposes as the basic clamping apparatus, but also for shifting a load from side-to-side.
In both modifications, with the arms moving toward one another, so long as the movement of neither arm is hindered by an external body, a series connection exists between the motors which move these arms, which connection produces relatively high-speed low-power relative movement between the arms. When movement of either arm is hindered, however, for example by one of the arms engaging the side of a load, a parallel connection is produced between the motors, which connection prepares the motors for lower-speed higher-power relative movement between the arms during clamping. As will be explained, under all circumstances of arm movement, the proposed invention minimizes the amount of pumped fluid which is required to produce such movement.
These and other objects and features of the invention will become more fully apparent as the description which now follows is read in conjunction with the accompanying drawings.